Static discharge is a naturally occurring electronic phenomenon. Triboelectric charge, often referred to as static electricity, builds up in all materials to some degree, and is eventually discharged as the charge traverses a path toward an electrical ground. Static electricity buildup is strongest in insulative materials.
Circuit boards contain microcircuitry which can be easily damaged by electrostatic discharge of relatively small magnitude. Magnitudes as small as 50 volts can permanently damage these devices. For comparative purposes, to illustrate the extreme sensitivity of microcircuits to this phenomenon, a visible charge from a human hand to a door knob in winter will often exceed 10,000 volts. Thus, extreme caution must be taken in protecting such microcircuit components from electrostatic discharge.
To provide protection from electrostatic discharge for packaged articles, a principle of physics referred to as the Faraday cage effect is often employed. Electricity does not penetrate a conductive enclosure. The static electric charge will go around the enclosed space, seeking the path of least electrical resistance en route to ground. By surrounding a static sensitive article with a conductive enclosure, the article is shielded from electrostatic discharge originating outside of the enclosure.
In my co-pending U.S. patent application Ser. No. 07/336,733, I have disclosed a multiple-ply anti-static paperboard product for use in Faraday cage-type packaging of electrostatic charge sensitive articles, such as electrical components or electrical circuit boards. The multiple-ply paperboard product comprises a layer of high-carbon content paperboard sandwiched between two layers of anti-static (now defined as static-dissipative) material. Such multiple-ply paperboard is particularly useful in the manufacture of dividers, pads and walls of containers for packaging articles which are required to be shielded from exposure to static discharge.
The packaging material described in the above-identified application has been found to be very beneficial in the protection of articles against damage by static electricity, but in many instances, that material affords more protection against certain types of static electricity and less protection against other types of static electricity than is required for specific applications. Specifically, and as one example, many applications require only protection against static electricity generated internally of the package either by an electrical component or article rubbing against a portion of the package or by one portion of the package rubbing against another portion of the package. In applications of this type, there is no need for the electrical conductivity of the center ply of this multiple-ply anti-static material.
It has therefore been an objective of this invention to provide a multiple-ply package material which may be utilized in the manufacture of a container or portion of a container and which provides effective protection against damage to products or articles packaged in the container resulting from static electricity to which the package may be subjected.
One attempt at protecting static electricity sensitive articles against static electricity is disclosed in U.S. Pat. No. 4,659,958 to McNulty. This patent discloses a bag having two plies of anti-static (polyethylene) material and an electrically conductive fabric or mat embedded therebetween to provide a shield from electrostatic discharge for a bagged article. Because the bag is lined with an anti-static material, it affords protection against electrostatic charge generated internally of the package, but the bag does not, by itself, provide adequate rigidity for physical protection of a microcircuit component. In order to provide physical protection, the bagged article must be placed within another cell in a rigid container. This results in additional material and material handling costs requiring a bag enclosure, additional labor costs associated with bagging the component or article, and additional shipping costs due to reduced packaging density. Reduced packaging density is due to the fact that each cell of a container which must hold an article within a bag, rather than just the article itself, occupies more space than the article alone. Thus, for a container having a given volume, use of bag packaging necessitates larger size cells, resulting in the packaging of fewer articles per container.
Another technique for protecting static electricity sensitive articles against static electricity is disclosed in U.S. Pat. No. 4,623,594 to Keough. This patent discloses that a mixture of prepolymer and anti-static agent may be applied to a substrate, such as polypropylene fiber or paper or glass, and then cured to set the anti-static agent by contacting the mixture with electron beam radiation. Because the mixture is cured in situ, after application to the substrate, this technique is very limited in its application and, to date, has only been applied commercially to bag materials. Consequently, for physical protection, articles protected against static electricity by material treated in accordance with the disclosure of this patent must be enclosed within a bag, and that bag, if it is to be physically protected, must be packaged in another rigid container. This again results in additional material and material handling costs, as well as reduced packaging density.
Heretofore, anti-static materials have also been applied to a cardboard substrate, but that cardboard substrate-applied anti-static material, usually low-density polyethylene, has been rendered anti-static by being doped with a chemical anti-static additive, generally some form of amine. Unfortunately, amines have several undesirable characteristics when used to impart anti-static properties to packaging materials. First of all, amines do not render the material to which they are added permanently anti-static. Rather, that anti-static coating of material loses its anti-static property over a period of time. Otherwise expressed, that amine-doped material has a relatively short shelf life because the amines, trapped within the plastic, evaporate or gassify with time from the plastic or other material within which the amine is trapped such that the plastic loses its anti-static property. Secondly, the amine is very corrosive to many metals, including the metals from which many microcircuits are manufactured. Consequently, the amine, gassifying from the anti-static plastic, can, and often does, corrode and impair the electrical transmission characteristics of the component which the anti-static material is intended to protect. Additionally, the amines which escape from the anti-static plastic can, and often do, attack the polycarbonate boards upon which many microcircuits are applied. In the course of attacking the polycarbonate, the amine causes stress cracks, and ultimately failure of the board. Additionally, the amines contained in the anti-static plastic are humidity dependent, meaning that the atmosphere within which the amine-doped plastic is intended to afford protection to articles contained in a package of the amine-containing anti-static material must be used in an atmosphere having a certain minimum humidity level. In some atmospheres, such as those which are very dry as in dry areas of the United States, amine-containing anti-static plastic materials have no anti-static property.
It has therefore been one objective of this invention to provide a relatively rigid anti-static multiple-ply packaging material which contains no amines and which is permanently anti-static.